1. Field of the Invention
The present invention relates to a mixer circuit and a radar transceiver, and more particularly to a mixer circuit and a radar transceiver for use in electronic devices and microwave and millimeter wave communication devices for mobile communications and wireless communications.
2. Description of the Related Art
In recent years, there has been an increasing need to reduce the size and increase the power of microwave and millimeter wave communication devices. Since it is not easy to directly generate an oscillating signal at a high frequency such as a millimeter wave frequency, the transmitter circuit in microwave and millimeter wave radar devices generates an oscillating signal at a frequency lower than the transmit frequency and up-converts it to a frequency an n multiple of the original frequency by use of a frequency multiplier (where n is a natural number equal to or greater than 2). The up-converted signal is then transmitted through the transmitting antenna.
For example, in the transmitter of an automobile radar device, the local oscillator generates a 38 GHz oscillating signal, which is amplified by an amplifier and up-converted to 76 GHz by a frequency multiplier before being transmitted through the transmitting antenna.
In the receiver circuit of the radar device, on the other hand, the radio frequency signal (RF signal) having a frequency fRF received through the antenna is down-converted to an intermediate frequency signal (IF signal) at a frequency fIF by a mixer driven by the local oscillator signal (LO signal) having a frequency fLO generated by the local oscillator.
In the case of a mixer in an automobile radar receiver, for example, when the frequency fLO of the LO signal is 38 GHz, the frequency fRF of the RF signal is approximately (but not exactly) 76 GHz. Therefore, the receiver mixer outputs the mixing component at the frequency fRF−2*fLO as an IF signal. That is, the RF signal is down-converted to the IF signal (having a frequency fIF) by mixing the RF signal with the LO signal having a frequency (approximately) one-half of the frequency fRF Of the RF signal. In this case, the radar device requires two antennas, namely, a transmitting antenna and a receiving antenna.
One such mixer circuit employs a rat race circuit (or hybrid ring) that includes first and second antiparallel diode pairs. The mixer circuit down-converts the RF signal to an IF signal by mixing the RF signal with the LO signal. (The IF signal has a frequency equal to the difference between the frequency of the RF signal and the frequency of the second harmonic of the LO signal.) It has been suggested that this mixer circuit can be used as both an up-converter and a down-converter. (See, e.g., JP-A-9-74316, paragraphs 0045 to 0053, FIG. 5.)
Another exemplary mixer circuit employs a rat race circuit that allows the mixer to be used for both transmission and reception when the frequency of the RF signal is not an n multiple of the frequency of the LO signal (where n is a natural number equal to or greater than 2). (See, e.g., JP-A-1-227553, lower left column on page 324, to lower right column on page 325, FIG. 4.)
Two other exemplary mixer circuits are adapted for use when the frequency of the RF signal is substantially equal to (not an n multiple of) the frequency of the LO signal. One uses a rat race circuit as a balun, while the other uses a Marchand balun. (See, e.g., JP-A-8-265048, paragraphs 0004 and 0007, FIGS. 9 and 10.)
Yet another exemplary mixer circuit includes a pair of antiparallel diodes having different junction sizes and hence different impedances. This circuit receives on its input terminal an IF signal and a signal having a frequency equal to one-half of the frequency of an LO signal and outputs on its output terminal an RF signal and the LO signal. Since this mixer circuit does not exhibit as much LO signal reducing effect as conventional mixer circuits, the output LO signal has a higher power. (See, e.g., JP-A-2004-166260, paragraphs 0041 to 0051, FIGS. 1 to 4.)
Further, one exemplary radio communication device adapted to operate at 30 GHz and higher frequencies includes an antiparallel diode pair having first and second terminals. In operation, a local oscillator signal (having a frequency fLO) is input to the first terminal and an information signal (having a frequency fIF) and a DC bias are applied to the second terminal, with the result that the n-th harmonic of the LO signal (i.e., n*fLO) and the mixing components at the frequencies n*fLO±fIF are produced at the second terminal, where n is a natural number equal to or greater than 2. (See, e.g., JP-A-2005-295097, paragraphs 0017 to 0019, FIG. 1.)
However, in order that a transceiver employing a conventional mixer circuit as described above function with only one antenna (for cost saving purposes), the transceiver must include a switch or a circulator to switch between the transmitter (or transmit mode) and the receiver (or receive mode). However, inserting such a component between the transceiver and the transmitting/receiving antenna may result in reduced transmission power and an increased noise factor.
Furthermore, this also prevents the transceiver from transmitting and receiving signals concurrently.